An enzyme-linked immunosorbent assay (ELISA) for monkey and human Interphoto-receptor Retinoid-Binding Protein (IRBP) was used to quantitate IRBP in normal and diseased human retinas and retinoblastoma tumors. IRBP levels were uniformly low in retinas from human cases of hereditary retinal degenerations even in areas in which photoreceptors remained. IRBP was present in several retinoblastoma tumors examined. The amino terminal sequences of monkey and bovine IRBPs were extended to over 30 residues each. The major monkey sequence had an additional 5 amino acid residues at its amino terminus not observed with bovine IRBP, although the sequences showed extensive homology. The amino terminal sequence of human IRBP was identical to that of the monkey, and the two sequences were present in equal amount.